The investigation into allegations that pornography was found on a computer from the office of deputy prime minister Damian Green could call Cornwall's top police officer as a witness.

The Cabinet Office inquiry, led by Sue Gray, Whitehall’s head of propriety and ethics, is examining a potential breach of the ministerial code, which states that “holders of public office” should be “truthful” and “accountable for their decisions”.

The allegations were first made public last week by former Metroplitan Police Assistant Commissioner Bob Quick as initially reported by the Sunday Times, in relation to a 2008-09 probe into leaked information.

It emerged this weekend that Sir Paul Stephenson, the Metropolitan Police Commissioner between 2009-11, said he was briefed about the claims but regarded them as a "side issue". Stephenson was appointed acting commissioner of the Metropolitan police in December 2008. He was fully briefed on all aspects of the original investigation.

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Mr Green, who is Prime Minister Theresa May's second-in-command, has said: "I reiterate that no allegations about the presence of improper material on my parliamentary computers have ever been put to me or to the parliamentary authorities by the police.

"I can only assume that they are being made now, nine years later, for ulterior motives."

It is understood that Green considers the computers involved were used by a number of people but neither he nor — as far as he is aware — his staff had downloaded any pornographic material.

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Other senior police officers were involved in the investigation, including Cressida Dick, who was Quick’s deputy at the time of the investigation and is now the Met chief. She told a parliamentary hearing last week she would help the Cabinet Office.

One of the other potential witnesses is Shaun Sawyer, now chief constable of Devon and Cornwall Police, who was involved in the inquiry. The Times reported that "Mr Sawyer’s press office said he was ready to co-operate".

Scotland Yard could be asked to produce case files, decision logs and intelligence reports from the 2008 and 2009 investigation for the Cabinet Office inquiry.

A Devon and Cornwall Police spokesman said: "Chief Constable Shaun Sawyer has not been approached by the Cabinet, however, if he was they would have his full co-operation. At this stage it would be inappropriate to comment further."